The present invention relates to a differential amplifier, particularly to an instrumentation preamplifier or sensor differential amplifier. Such amplifiers preferably have constant gain over a wide temperature range. The present invention also relates to an asymmetrical differential amplifier with differing gains on its two inputs.
In analog circuitry, signals are often represented by differential signals, e.g. voltages or currents. A differential signal may be described by a common mode component and a differential component. The common mode component may be looked upon as the average of two signals whereas the differential component is the difference between the same signals. Preferably, a differential amplifier has a high immunity to common mode changes or noise (high common mode rejection) and is therefore often preferred compared to a single end amplifier. Different applications place different requirements on a differential amplifier, for instance very precise gain is preferred for instrumentation applications whereas for sensors constant gain over a wide temperature range is more important than accurate gain at one temperature. The temperature range at constant gain preferably extends up to temperatures used for sensor equipment in which thermal noise has to be eliminated.
FIG. 1 illustrates a known fully differential amplifier using two single ended amplifiers and a resistor network of three resistors. The latter determine the gain of the complete device. The input and output common mode signals are at the same level while the differential input signal is amplified depending on the ratio of R1 and R2. The precision of the gain is determined by the precision of the resistors and by the finite gains of both amplifiers. By matching resistor characteristics and keeping the gain of the amplifiers as high as possible, the gain may be made independent of temperature over a large range. WO97/36371 describes an improvement of this known device in which switching is used to eliminate resistor mismatch.
There are several problems with this known device. First of all the gain is limited by the resistor ratio obtainable. Secondly, integration onto a single chip requires a lot of chip area for the resistors and amplifiers with Miller capacitors and current-consuming output stages to drive the resistors. In the case of a preamplifier for sensor applications, two stages are normally used. The first stage is the sensor itself with its bias circuit followed by a second stage fully differential amplifier, each stage of this chain contributing to the noise.
A single ended OTA (Operational Transductance Amplifier) is known from the article by Wang et al., Analog Integrated Circuits and Signal Processing, 8, pages 21-35 (1995), Kluwer Academic Publishers, Boston (USA), entitled xe2x80x9cPartial positive feedback for gain enhancement of low power CMOS OTA""sxe2x80x9d. The OTA includes an internal active load for gain and bandwidth enhancement. It comprises an input transistor pair whose sources are connected to a bias current source. Differential input currents are mirrored by an active mirror which presents a different mirror ratio for common mode and differential inputs. The desired gain depends on the transistor size ratio. One of the active load currents is mirrored by a further current mirror. The output current is formed by the difference between this last mirrored current and the other output of the active current mirror. An optimised sizing of the active current mirror allows a gain and bandwidth increase without increasing the power consumption. The whole amplifier can be considered like a single stage amplifier with only one dominant pole and therefore no frequency compensation capacitor is required.
EP-A-554743 describes a fully differential amplifier with common-mode stabilitv enhancement. In order to get a higher pole in common mode versus differential mode, an active load is used whose scheme is similar to the active load of the Wang amplifier. The active load acts like a current source with a very high differential input impedance. The amplifier can therefore be considered as a two stage differential amplifier which may imply the use of a Miller capacitor for stability. Inversely, for common mode signals the active load acts as a low impedance. In this case, the amplifier acts as a single stage amplifier, which has no stability problem. Common mode regulation circuitry is provided which consists of a single ended amplifier and two resistors. It is necessary to set the common mode output DC voltage. If this is not done, the common mode output voltage may be too low, pushing the input transistors into the linear region. If the voltage is too high pushing the output transistors into the linear region. Accordingly, it is necessary to provide a feedback circuit to regulate the DC output voltage. The junction of the resistors of the feedback circuit is known as the common mode summation point and gives a measure of the DC common mode output voltage. the feedback control should operate in such a manner that all transistors are kept in the saturation region.
U.S. Pat. No. 3,991,380 describes a complementary field effect transistor differential amplifier which feeds back output voltages from output nodes 12, 22 to common summation nodes xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d. Transistors P-3, P-4, N-3, NA are coupled together in pairs to form resistor networks the centre points of which provide the summation points.
The book xe2x80x9cDesign of Analog Integrated Circuits and Systemsxe2x80x9d by K. R. Laker and W. M. C. Sansen, McGraw-Hill International editions, 1994, provides many useful comments on and examples of the design of OTA""s and of common mode regulation feedback circuits.
An object of the invention is to provide a differential amplifier which allows integration onto chip in a compact and efficient manner.
Another object of the invention is to provide a differential amplifier with a constant gain over a wide temperature range.
A further object is to provide a differential amplifier with a simpler circuit design.
Still a further object opf the present invention is to provide a differential amplifier which is stable independent of the load capacitance.
Yet another object of the present invention is to provide a differential amplifier which may be constructed using various technologies.
Still another object is to provide an asymetrical amplifier.
The present invention provides a differential amplifier comprising: an input branch for receiving differential and common mode input signal components; an output branch providing differential and common mode output signal components; and the differential amplifier being adapted to set a relationship between the magnitude of the common mode output signal component level of the amplifier with respect to the magnitude of the common mode input signal comonent level to the input branch so that the common mode output signal component level intrinsically follows the common mode input signal component level as a common mode follower without using feedback of the common mode output signal component. The input and output branches are connected in parallel. The amplifier does not use feedback control of the common mode output signal component to or from a common summation point. The common mode output signal component voltage Vcom of the amplifier is at least approximately represented by:
Vcom=xcex1Vin+k 
where Vin is the common mode input signal comonent voltage and xcex1 is within the range 0.5 to 1.5, more preferably in the range 0.7 to 1.3. The amplifier is powered by voltage source of voltage Vs and k may be xc2x150% of Vs or of less absolute magnitude. The value of k may be within 300 mV of a set value of k. The differential gain precision at room temperature may be 20% or less, preferably 5% or less, most preferably 1% or less. The differential gain drift between xe2x88x9220xc2x0 C. and 250xc2x0 C. is preferably 30% or less, more preferably 20% or less, most preferably, 5% or less. The differential gain may be set to between 1 and 50 at room temperature by the design of the amplifier.
The differential amplifier may further comprise an active load circuit, the active load circuit linking the input branch to the output branch and having a larger differential gain than common mode gain. The active load circuit may comprise two current mirror circuits linking the input branch to the output branch, wherein the current mirror circuits each have a differential gain different from its common mode gain. The active load circuits are designed to provide the same current densities in the transistors of the input and output branches. This improves the temperature response of the amplifier. The amplifier is preferably implemented using active devices of the same type and manufacture, e.g. transistors and that by matching these transistors the differential gain is constant to within 1 dB, more preferably to within 0.5 dB, and most preferably to within 0.2 dB over a temperature over a range of at least 100xc2x0 C., more preferably at least 150xc2x0 C. nd most preferably over 200xc2x0 C.
The amplifier described above is preferably used as a preamplifier. The amplifier may be constructed in a variety of technologies, e.g. MOS field effect transistors, bipolar transistors, SOI. One particular advantage is that it does not require passive components so that it may be integrated easily and compactly onto a single chip.
The present invention also includes an asymmetrical differential amplifier with at least one output and at least two inputs, preferably for use with the differential amplifier mentioned above, wherein the gains on each of the two inputs are different One of the inputs is preferably split into two inputs, the second input being for application of a differential signal opposite in sign to the signal applied on the one input.
The present invention will now be described with reference to the following drawings. Further embodiments of the present invention are detailed individually in the dependent claims.